In your case, you could simply select option 2, above. This would be a good option if this is your first install. It will wipe Windows and delete your data.

The third optin, "Something else," allows you to delete existing partitions, create new partitions, and format partitions. This could be a choice for you as well, if you want a little more control over your disk.

If you're unsure about partitioning, go with option 2.

Also, be ready for a learning curve, as with any new Operating System. However, you will really appreciate being able to use your computer without the viruses.

Yes ofcourse you can.
And to clear your hard drive you do not need an external tool.
You just have to download ubuntu iso, write it to a disk,boot from it, and when installing choose the option-
wipe everything and install

You are now in the place that I was in about a year ago! It can be a bit tough at times but stick with it - it's worth it. But start with a plan, even if it's just a simple one in your head. List what you need from your Windows disk, for example:

Your CV

All business letters/spreadsheets/databases/ebooks/manuals/downloads

All photos & music

Important emails that are not also with the email service provider

Next, decide where are you going to put them. Depending on the space required, you could use a free account with dropbox - 2+GB. Another 15GB with Google drive; 1TB (yes, that's 1000GB or 1000000MB) for your photos with flickr. If required/preferred, use an external USB drive, as they are cheap enough now. Boot from a Linux DVD and do the copying under Linux. You will be able to scan the files for viruses under Linux later. Then follow Mitch's instructions to Replace Windows.

You will experience niggles and doubts but stick with an LTS (Long Term Support) release of Ubuntu so you don't get the recently-added-but-not-thoroughly-tested enhancements and additions. Find alternative programs to allow you to what you do with Windows. There's a lot to choose from sometimes but you can ask advice on the forums like these. Good luck!

I would recommend, if you have the space on your hard drive, keeping your Windows installation and creating new swap and ext4 partitions for your new Ubuntu install. In other words, choose "Install Ubuntu alongside Windows" when asked, being that you surely have some documents, music, photos &c. in your Windows install that you'd like to save. Once you have successfully installed Ubuntu, you can copy over anything you want to save from your Windows partition into your ext4 partition. (Ext4 is the filesystem used by Ubuntu; Windows uses NTFS, a proprietary Microsoft filesystem.) After you've finished moving the files you want to save, you can remove your Windows partition and enlarge the ext4 space, by booting from a live USB or DVD and using the gParted tool, which comes with Ubuntu by default. You may need to update your GRUB (GRand Unified Bootloader) after resizing your ext4 partition, and I've used instructions for doing so from http://ubuntuforums.org/showthread.php?t=1581099 with unqualified success. I also migrated to Ubuntu 12.04 after having used Windows exclusively my entire life, and I cannot express how much more satisfied I am with Ubuntu's performance.